Hydraulic head applications of flowmeter logs in karst aquifer studies Fred Paillet Geosciences...
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![Page 1: Hydraulic head applications of flowmeter logs in karst aquifer studies Fred Paillet Geosciences Department University of Arkansas.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649e405503460f94b3250f/html5/thumbnails/1.jpg)
Hydraulic head applications of flowmeter logs in karst aquifer studies
Fred PailletGeosciences Department
University of Arkansas
![Page 2: Hydraulic head applications of flowmeter logs in karst aquifer studies Fred Paillet Geosciences Department University of Arkansas.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649e405503460f94b3250f/html5/thumbnails/2.jpg)
FLOW LOGGING
Flowmeter underpump indicatesflow zones and relative contribution
But estimates oftransmissivity arehighly local and notrepresentative of the flow path
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AltonabedrockboreholesExample whereclosely spaced boreholes yieldcompletely different estimatesof T for thesame solutionedbedding planes
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Using flow log data to measure hydraulic head
• Formulate a two variable inversion• Obtain two flow profiles under different
conditions• Usually ambient and pumping• Measure difference in open-hole water level• Model flow using specified T and h values • Vary T and h until match BOTH profiles
simultaneously
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TWO STEADY FLOW PROFILES USUALLY AMBIENT AND STRESSED
Productive karst – Low Q drawdown same size as head differences so that drawdown is the same order of magnitude as the naturally
occurring hydraulic head differences, biasing T measurements.
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Example where ambient head differences have a major influence on the detection and characterization of flow zones in a karst aquifer. Presence of
major flow zones masked by hydraulic head differences.
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Ambient flow – no flow tolowest zone
Pumping flow – no drawdownon middle zone
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Nuts and boltsofFlow log interpretation
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SUBTRACTION OF INFLOWS METHOD Remove head influence by elimination of variable but also throw out any attempt to infer hydraulic head
for each flow zoneRef: Molz et al, 1988, WRR
Depth Amb Ambinflow
Pump Pumpinflow
Diff ofDiffs
% ofTotal T
Above Below Diff Above Below Diff Δdiff Δ as %pump
m l/min l/min l/min l/min l/min l/min l/min %
16.8 0.00 -1.20 1.20 10.00 6.00 4.00 2.80 28
32.0 -1.20 -0.05 -1.15 6.00 1.50 4.50 5.65 57
39.8 -0.05 0.00 -0.05 1.50 0.00 1.50 1.55 15
Verify 0.00 10.00 10.00 100
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Paillet (WRR, 1998)model
An alternate approach is to usea flow model to simultaneously fit models to ambient and pumped flow profiles giving directmeasurements of T and h
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BOREHOLE FLOW MODELING Yields direct estimates for both T and h
MODEL BOTH HEAD AND TRANSMISSIVITY FLOW ZONE TRANSMISSIVITY ZONE HYDRAULIC HEAD
16.8 m 2.0 × 10-5 m2/s 5.95 m below TC32.0 4.0 × 10-5 6.8739.8 1.3 × 10-5 6.87
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Flow logs in off-line drainage wellsSolution horizons in gypsum rubble
aquifer
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VERIFICATION OF FLOW MODEL ESTIMATES OF WL
Piezometers available in the vicinity of two of the logged drainage wells
PIEZ DEPTH m MODEL DEPTH m WL PIEZ m BGL WL MODEL m BGL
WELL 2A
OPEN HOLE OPEN HOLE 4.72 4.72
12.0-15.0 18.0-20.0 4.32 4.54
30.0-32.0 36.0-38.0 4.87 4.91
43.0-46.0 43.0-46.0 4.82 4.91
WELL 5
OPEN HOLE OPEN HOLE 2.83 2.83
4.2-6.2 8.0-12 2.81 2.71
12.3-15.2 20.0-22.0 2.81 2.84
21.2-24.2 28.0-30.0 3.39 3.45
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Trolling EM FlowmeterHead values indicate an aquitard near 40 m in depth and little vertical head
gradient above 40 m
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Wirelinepackersystem
Suitable for use asa standard probe run with other probes during normal wellloggingoperations
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Single packer setting – Convert to readings between stationsSite directly above pumped aquifer and had assumed a strong vertical gradient in efforts to monitor heavy metal contamination. Packer data shows lateral drainage by karst bed and negligible downward gradient below 200 feet.
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FLOWMETER CROSS-BOREHOLE TESTSMonitor the propagation of drawdown outward along flow paths by measuring the evolving flow
regime in an adjacent borehole
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Single fracture experiment toverify model predictions where there is a known analytic solution
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Ambler PA cross-hole test
• Solution on bedding plane connects boreholes• Boreholes 30 m apart• Pumped well – T = 350 m2/day• Observation well – T = 250 m2/day• Pump rate = 23 liters/min• Model response with T = 300 m2/day• Storage coefficient (S) the only variable• Test duration – 1 minute pulse
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T = 300 m2/dayS CONTROLS AMPLITUDE
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0 60 120Obs flow
S = 5exp-5
S = 3exp-5
S = 1exp-4
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A more ambitious use of the cross-hole model
Leakage between fractures• Two-bedding planes• Pumped well has upper plane cased off• Pump only from lower zone• Measure flow between zones in obsv well• Expect pull down flow from upper to lower
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LEAKAGE BETWEEN ZONES
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Effect of Leakage on DownflowHead decay time = 1/L in minutes
0 2 4 6 8 10 12 14 16 18 20-0.3
-0.2
-0.1
0
0.1
L = 2.50
L = 0.50
L = 0.25
L = 0.10
L = 0.05
L = 0.00
Time in minutes
Flo
w in
ga
llon
s p
er
min
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LEAKY AQUITARD TEST
• Two high T karst aquifers• Upper intersects canal• Pump from well completed in lower• Measure borehole flow between aquifers• Is aquitard between them leaky?• If no leakage – pumping induces down flow that
steadily increases• If very high leakage flow shows short downward
pulse that relaxes over time
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Flow schematic
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S FLORIDA KARST T1= T2 = 50,000 ft2/day
-150.000
-100.000
-50.000
0.000
50.000
0.00 5.00 10.00 15.00 20.00
Time in minutes
Flo
w i
n l
iter
s/m
in
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Head decay time = 1/0.05 = 20 minutes
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Conclusions• T derived from flow logs is highly local• h derived from logs or packers denotes large-scale flow
path• Two-variable interpretation can be applied to suitable pairs
of flow logs to give T and h• Derive estimates of head on flow paths in open boreholes• Results obtained as part of the routine logging process• Values not as accurate as obtained with packers – but with
much less expense and effort• Used as stand-alone data or to prepare for more effective
straddle packer program